1. Field of the Invention
The present invention relates to an image processing apparatus, spatial frequency conversion method for the image processing apparatus and recording medium. The invention particularly relates to an image processing apparatus for performing calibration of spatial frequency conversion on image data having been read from a double sided document sheet, a spatial frequency conversion method applied to the image processing apparatus, and a recording medium storing a program for execution of the spatial frequency conversion on a computer.
2. Description of the Related Art
Conventionally, as image reading units to be used for an image processing apparatus or the like, image reading units that perform so-called “skimming thorough” are known (refer to Japanese Patent Laid-Open No. 2001-285595, for example). With this image reading unit, original document sheets are conveyed one by one by an original document sheet conveying unit onto a glass document table, and the original document sheets are exposed by an exposure unit fixed on the convey path of the conveying unit so that images are read.
Further, there are known image reading units provided with two image reading sections so as to read the front and back sides of an original document sheet (also referred to as an original) in one-time conveyance for improvement of the image reading speed (Refer to Japanese Patent Laid-Open No, 2004-187144, for example).
Such image reading units that read the front and back sides of an original in one-time conveyance include a type having a reduction optical system using a CCD (Charge Coupled Device) line sensor or reduction lens and a type having an equal magnification optical system using a contact type image sensor CIS (Contact Image Sensor). Such an image reading unit causes difference between the front and back sides of an original in the spatial frequency response characteristic and density characteristic. When a difference in spatial frequency response characteristic is caused, a difference is caused between the front and back sides in contrast and the degree of sharpness. Further, even when the same type of an optical system is used to read the front and back sides of an original, a difference in assembling of respective components, spectral characteristic, and the like between optical systems likewise cause a difference in reading characteristic between the front and back sides of an original.
Regarding the spatial frequency response characteristic among the differences in such reading characteristics, there are conventional image reading units that perform adjustment of a reading optical system and/or smoothing filtering processing of read image data (Refer to Japanese Patent Laid-Open No. 2002-218248, for example). Such an image reading unit reduces irregularity in the spatial frequency response characteristic in the same side of an original, and also reduces the difference in spatial frequency response characteristic between the front and back sides of the original.
Further, in order to reduce the irregularity in the spatial frequency response characteristic in the same surface, there are proposed methods that change the spatial frequency response characteristic in filtering processing, corresponding to the position in a surface (refer to Japanese Patent Laid-Open No. 2008-098921, for example).
However, the conventional image reading units disclosed in the Japanese Patent Laid-Open No. 2002-218248 and Japanese Patent Laid-Open No. 2008-098921 require a special chart for calculation of spatial frequency response characteristic that is necessary for calibration. Further, a unit that reads the chart and analyzes the spatial frequency response characteristic may be necessary. Thus, the environment for executing the calibration of spatial frequency response characteristic is limited. In a case of using a special chart, the spatial frequency response characteristic necessary for calibration is in general calculated at the time of production at or shipment from a factory.
Although there is no problem as long as an image reading unit is always stable, various cases are possible as well as temperature and humidity in terms of the environment where an image processing apparatus is used, and a case may occur where calibration has bad effects on output image data due to variation in usage environment and secular change. In particular, in a case of an image reading unit that reads the front and back sides of a sheet in one conveyance, it is possible that the variation amounts due to the usage environment and secular change are different between two devices, which causes a problem that adjustment at the shipment from a factory does not achieve sufficient calibration. Therefore, a method is desired that calculates a spatial frequency response characteristic/characteristics necessary for calibration, without requiring a dedicated chart or device under any usage environment and at any time.
The present invention has been developed in view of the above-described problems. An object of the invention is to provide an image processing apparatus, spatial frequency conversion method for the image processing apparatus and program that do not require a dedicated chart or device under any usage environment and at any time and are able to calculate a spatial frequency response characteristic/characteristics necessary for calibration.